1. Field of the Invention
The present invention relates to a sheet-like light source device; in particular, a sheet-like light source device for liquid crystal display devices.
2. Description of Related Art
In recent years, the flat panel display industry has rapidly developed. To improve the performance of the displays, the industry has devoted significant efforts to improve and design the structure of the respective components of the display devices, among which the sheet-like light source device of the liquid crystal displays has been improved to provide a light source with a highly uniform luminance distribution of light.
Because the liquid crystal itself is not luminous, it is necessary to add a backlight source (generally, a fluorescent tube) to the rear of a liquid crystal display panel to project light through the display panel. To this end, the light emitting from the light source passes through a polarizer and the liquid crystal in sequence, thereby the arrangement of liquid crystal molecules of the liquid crystal display is changed by voltage. Accordingly, the angle of the light passing through the liquid crystal is altered, resulting in a non-uniform luminance distribution of light on the liquid crystal panel.
In general, the sheet-like light source device is categorized into an edge-light type and a bottom-light type, depending on the position where the sheet-like light source device is disposed. The edge-light type sheet-like light source device is generally used in the small-and-medium displays such as notebook PC displays, PDA displays and cell phone displays. A key component of the edge-light type sheet-like light source device is the light-conducting plate which is provided primarily for directing light from the edge to the visual line. In other words, the light projected from in the edge is uniformly directed to and distributed on the whole light-conducting plate where the visual line is attainable.
A conventional sheet-like light source device has a housing (frame) for holding a light-source unit, a light-conducting plate and a print circuit board. The light-conducting plate is embedded in the center of the housing while the light source is disposed on a lateral side of the housing. A space remains between the light source and the light-conducting plate to facilitate transmission and reflection of light. To prevent the lamp (or the CCFL) within the sheet-like light source device from being cracked by the impacts caused by the motion of the light-conducting plate, a rectangular convex portion 100 is provided with the light-conducting plate, and also, a rectangular concave portion 201 corresponding to the rectangular convex portion 100 is disposed on a housing 200, as shown in FIG. 1. Thus, the light-conducting plate is embedded and fixedly disposed. However, it has been found that light from the light source always concentrates in the right-angled position 100a or 100b of the rectangle when light projects to this light-conducting plate. The concentration phenomenon generates because the light is partially reflected and partially transmitted to the frame when light reaches the convex portion of the light-conducting plate. In addition, the light transmitted to the housing is reflected back to the lateral portion of the light-conducting plate, and the leaking light is repeatedly reflected between the housing and the light-conducting plate. As such, light is concentrated in the position where the convex portion is disposed.
To solve this drawback, U.S. Pat. No. 5,988,827 disclosed a method for changing the folded angle of a convex portion of a plastic frame (or retention member) in a sheet-like light source device so as to improve the reflection of light. As shown in FIG. 2, the right-angled portion is changed to become a rounded portion to disperse the light reflected to the convex portion. However, not only the angle of the light-conducting plate need to be changed, the concave portion of the frame (or retention member) has to be rounded to match the change of the angle of the convex portion 300. The change illustrated above in fact wastes time in manufacturing process and is not suitable for mass production.
U.S. Pat. No. 6,561,664 disclosed another method, in which a black tape 400 is adhered to a concave portion 201 of a frame 200, as shown in FIG. 3. Thus, most of the leaking light reflected directly from the position where the convex portion of a light-conducting plate is disposed can be absorbed. Even so, the black tape is easy to fall off, thereby impairing the engineering reliability.
Therefore, it is desirable to provide an improved sheet-like light source device to mitigate the aforementioned problems.